In the field of high power amplifiers, especially for audio use, the amplifiers are notoriously short lived. This phenomenon is due in large part to the applications to which high power audio amplifiers are put. For example, many such amplifiers are used in the live performance of popular and rock and roll music. It is common to purposely overdrive the amplifiers to clip and distort the output signal, thus emphasizing the signal harmonics at the expense of the signal fundamental. Also, in such uses the amplifiers are generally operated at full output for extended periods. This tends to create high operating temperatures.
In prior art amplifiers, the last driver stage that drives the output transistors turn the output transistors on when they are turned on themselves. They either draw current through a resistance across a MOSFET to apply gate voltage, or they draw current through a base to apply base current from the device that is driving. In prior art designs, if the drivers draw more current they cause the output transistors to draw more current, in cascade fashion, and lead to thermal runaway. Negative feedback can be used to counteract the thermal runaway problem, but this introduces further complications by altering the output signal characteristic.
Most solid state amplifiers are not designed to overdrive and distort without severe problems in addition to thermal runaway. The common result is either damage to the amplifiers or destruction of the loudspeakers. To protect the amplifier, many manufacturers have added protection devices, termed energy limiting or VI protection. These devices generally limit the output current at a threshold set by the recent value of the output signal voltage and current. Although this would appear to be a useful technique, it also leads to further problems. At high output levels, the signal current is constantly being limited in erratic fashion, leading to wild fluctuations in the output impedance (the ratio of V/I). This causes impedance mismatches with the loudspeakers.
The output impedance of a typical amplifier in the operating mode can be as little as 7 milliohms in series with less than 3 microhenries. The impedance can jump to several thousand ohms. When the loudspeaker cone being driven by the amplifier relaxes, the voice coil moving through the magnetic gap can generate a substantial back current. If the output impedance of the amplifier is sufficiently high, the result is a voltage signal which drives the amplifier. This flyback pulse returns the inductive energy of the load to the opposite polarity power supply of the amplifier with respect to the side that produced the excessive output. The result is a rasping, popping sound which is very disturbing.
Furthermore, the VI protection circuits for many amplifiers will start putting a direct current into the speaker. They often generate an off-center DC voltage at the output when they are being overdriven. The direct current output causes the loudspeaker cone to shift from its nominal resting position, and thus limits the further excursions of the cone required to transduce the output signal.
The typical response to these drawbacks of the prior art amplifiers has been to add further circuitry to alleviate these negative effects. As a general result, the circuitry of audio power amplifiers has become enormously complex, leading to very high numbers of components and attendant high costs. This also leads to more difficulties in servicing of damaged amplifiers.
Due to the nature of live performance popular and rock and roll music, the power amplifiers also suffer from physical abuse. Many music groups perform on tours, and the sound equipment is set up and taken down almost daily. Considering the skill levels of some road crews, this situation is rife with opportunities for misconnections (i.e., connecting the amplifier output to the input jack), ground faults which input 60 cycle hum directly into the amplifier, direct short circuits of the output, and the like. No prior art amplifier design can withstand such treatment.